This invention relates to the lamination of plastic materials in general and more particularly to a method for laminating an expanded core to flat sheets of materials which do not easily bond. Certain plastic materials are particularly useful in chemical applications because of their excellent chemical resistance. A particular example is polypropylene which has bound much use in chemical tanks, plating tanks, tanks for aeromatic fluids and gasoline. The typical procedure is to weld together a number of solid polypropylene plates to, for example, a thickness of one and one half inches. Such an arrangement results in a heavy sheet of material which is costly.
A method of expanding plastic materials to form relatively large expanded panels is disclosed in U.S. Pat. No. 4,113,909, granted to Donald R. Beasely and assigned to the Norfield Corporation. The technique used is one developed by Walter Smarook and described in more detail in a series of patents referred to in the aforementioned U.S. Patent. In this method a sheet of plastic material is placed between platens containing a plurality of vented openings. The platens are heated and the plastic material adheres to the plates because of hot tack adhesion. The platens are moved apart and due to the venting by the vented openings, the plastic takes on a new internal geometry, and expanded panels results after the necessary cooling. The panel is structurally strong and of light weight with honeycomb like internal structure, i.e., a plurality of voids surrounded by rib members of polypropylene. While the structural strength and light weight of the panels make them ideal for use in chemical tanks or the like, the honeycomb like internal structure could result in leakage. For this reason it is desirable to bond flat sheets of material such as polypropylene to the polypropylene core. However, it is well known that polypropylene does not bond easily and there are no known bonding agents which can be used to achieve a structural bond. Polypropylene which is manufactured in relatively thin sheets is formed into thicker sheets by a heating process which fuses the layers together. Such a heating process is the only known way to bond polypropylene to itself and has been used in the welding of corners in tanks. Although spot welding of the polypropylene sheets to the core might be possible, the bond which is obtained in not reliable and may result in leakage from one void in the core to another. Such leakage, particularly in the use of volatile material can cause serious problems. Thus, a better method of bonding is required.